You don't even need a complete asteroid. You only need a few hundred or maybe a few thousand tons of shielding material. You could get that from any nearby (r

You don't even need a complete asteroid. You only need a few hundred or maybe a few thousand tons of shielding material. You could get that from any nearby (relatively speaking) asteroid-like body with a reasonable delta-v and a surface covered by loose regolith. Mars's moon Deimos might be the best candidate -- delta-v between Deimos and Earth-Moon L4/L5 is about 3800m/s [1], and there's no need to move the bulk of your shielding lower into the gravity well on either end. In this scenario, you'd want to use something like an Aldrin Cycler [2] to provide radiation shielded transport between Earth and Mars. This isn't exactly the same thing as hitching a ride on an asteroid, but it's the same basic principle. You'd definitely need nuclear electric or maybe solar electric propulsion to make such a scheme work, though. [1] https://en.wikipedia.org/wiki/Delta-v#Delta-vs_around_the_Solar_System https://en.wikipedia.org/wiki/Delta-v#Delta-vs_around_the_So... [2] https://en.wikipedia.org/wiki/Mars_cycler https://en.wikipedia.org/wiki/Mars_cycler